This 2TB NVMe SSD is a great Steam Deck upgrade for £155
Corsair's MP600 Core Mini SSD is a solid option for upgrading a PC handheld - devices like the Steam Deck, ROG Ally and Legion Go and so on. It's a PCIe 4.0 NVMe SSD, so it offers slightly faster speeds on the latter two devices than on the Steam Deck, but it's compatible with all three major options. It originally retailed at £230 for the 2TB model, but now it's at a much more reasonable price: £155.